Generally, a solar cell refers to a generator which absorbs light to generate and transfer electricity to the outside, and has attracted attention as a type of reproducible power generation system which does not use fossil fuel.
Solar cells may be generally classified into semiconductor type solar cells and dye-sensitized solar cells according to a principle of power generation. Semiconductor solar cells refer to photovoltaic cells, which employ photovoltaic power generated in a P-N junction between a P-type semiconductor and an N-type semiconductor. The semiconductor type solar cells may be classified into a silicon solar cell, a compound semiconductor solar cell, and an organic solar cell according to materials used.
Although some solar cells exhibit significantly high conversion efficiency in laboratory tests, typical solar cells exhibit conversion efficiency ranging from about 10% (organic solar cell) to about 18% (silicon solar cell) in practice. For the solar cells, it is an important issue to improve conversion efficiency.
In recent years, various efforts have been made to improve conversion efficiency of the solar cell not only through development of new materials, but also through optimal use of light incident on the solar cell.
As part of such efforts, studies have been made to collect as much light as possible when sunlight enters the solar cell. In one method, an anti-reflection layer is formed on an incident plane, through which light enters the solar cell, to prevent reflection on the incident plane, and a surface of the incident plane is subjected to texturing to induce diffuse reflection. In another method, a reflective film is formed on a rear side of the solar cell in order to use light passing through the solar cell.
Meanwhile, when generating electric power, the solar cell employs light in a predetermined wavelength range depending on the type (material) of solar cell instead of employing all components of light entering the solar cell. For this purpose, the band gap of a solar cell material, the depth of a P-N junction, a coefficient of light absorption, and the like are selectively determined.
FIG. 2 is a graph depicting energy distribution of sunlight and spectral distribution of a silicon solar cell.
As shown in FIG. 2, sunlight generally consists of about 3% of UV light, about 43% of visible light, and about 54% of IR light. For a silicon solar cell, some light components in the wavelength range from about 570 nm of visible light to about 1,120 nm of near-infrared light mainly contribute to generation of photoelectric current, and the remaining light components, that is, 15-25% of green and blue light having wavelengths of about 570 nm or less and long-wavelength radiation energy of near-infrared light having wavelengths of 1,120 nm or more, do not contribute to power generation.
In order to use light in the wavelength range that does not contribute to power generation, a recent solar cell structure includes a stack of solar cells, which use different types of wavelengths for power generation. However, such a solar cell structure is manufactured through a complicated process, increasing manufacturing cost.
Therefore, there is a need for a new technique which allows use of a wide wavelength range of light incident on a solar cell for power generation.